If you take these things one at a time, I don't see any big problems. You'll have to count the number of required inputs/outputs and figure-out if you need some sort of "expansion". And, try to think about data-rates... You can update things like speed & temperature a few times per second, but audio requires thousands of samples per second, etc.
I'm not sure if GPS will give you enough short-term real-time information to work as a speedometer, especially at slow speeds.
Voltage & current should be no problem. The current is an indication of battery drain at any moment in time. Your motor is going to take whatever it needs or whatever it can get, depending on the situation. Usually, you can stall a DC motor (worst case heat/load situation) with full-voltage applied, it will pull the maximum current, and nothing bad will happen. But I suppose every motor is different.
Automatic shutdown should be OK as long as there is sufficient warning, allowing you to pull off the road. And, maybe a bypass button. Or maybe instead of automatic shut-down, a big warning light that tells you to stop and shut-down.
Of course, your music player is going to require either streaming audio, or local storage. The music player might need it's own Arduino, or a different processor.
An RF receiver/transmitter could be used for your garage door opener, but you might need a 2nd Arduino on the receiver (garage door) side.
Using a keypad access/security code, shouldn't be a problem.